1. Field of the Invention
This invention relates to antenna design and, more particularly, to biconical antennas employing equal-delay or Guanella baluns.
2. Description of the Related Art
The following descriptions and examples are given as background only.
The equal-delay or Guanella balun is one of the most common broadband transformer and balun topologies. Absent imperfections, the topology is pulse preserving and hence is frequently employed as a pulse transformer. It is often combined with antennas used for EMC testing, including the broadband wire-cage biconical antenna, as well as some implementations of the Impulse Radiating Antenna (IRA).
A particularly robust implementation of this topology, which is based on a pair of bifilar helical transmission lines is widely used with broadband wire-cage biconical antennas for Electromagnetic Susceptibility (EMS) testing from 30-300 MHz. This implementation differs from most equal-delay designs in that the electrical lengths of the constituent transmission lines are electrically long over most of the operating frequency range. The electrically long structure provides the necessary choking reactance in the absence of ferrite cores. The absence of ferrite is quite advantageous for sustained high power operation.
FIG. 1 depicts an embodiment of an equal delay balun with a generalized load to represent an antenna driven from a coaxial feed line. The shunt-series interconnection of the constituent transmission line elements provides a 1:4 impedance transformation, which is useful for broadband antenna matching. In the ideal case, in which the constituent transmission lines of the balun are equal in electrical length (θA=θB) and characteristic impedance (Z0A=Z0B), the operation of the equal-delay transformer/balun is frequency independent and maximum power transfer occurs when the impedance of the source (Zg) is ½ (Z0A=Z0B) and the impedance of the load (ZL) is 2(Z0A=Z0B).
However, imperfections in the balun tend to destroy the equal-delay nature of the device, and hence, its fundamental frequency independence. While it is reasonable to assume that the characteristic impedance of the two constituent transmission lines can be made essentially equal through precise manufacturing, it is nearly impossible to make the electrical length of the transmission lines perfectly equal. Even small differences in electrical length can cause dramatic variations in performance, thus preventing the equal delay transformer/balun from reaching idealized performance.
When the electrical lengths of the two constituent transmission lines are not commensurate (i.e., not perfectly equal), the balun tends to exhibit anomalies at odd-integer multiples of the average quarter-wave frequency of the two constituent transmission lines. When combined with a radiating structure (such as a broadband wire-cage biconical antenna), these anomalies manifest themselves in the antenna's response, and may involve undulations in the power transfer, peaks in the return loss of the system, and excitation of the common mode of the radiating structure.
Therefore, a need exists for a modification to conventional biconical antenna designs employing equal-delay baluns. Specifically, a need exists for a modification that would prevent the anomalies that necessarily occur in the equal-delay balun (due to imperfections in the balun) from manifesting themselves in the antenna's response.